Default "vp9" decoder didn't properly handle alpha (transparency)

[Mista_D]

FFmpeg doesn't seem to recognize VP9 encoded alpha channel file (yuva420p). It decodes it by default with VP8 and therefore fails to see alpha channel in WEBM.

This use case works well (decoder is forced to vp9), overlay is good, alpha is crystal clear:
ffmpeg -i udp -c:v libvpx-vp9 -i ov3.webm -filter_complex [0:v][1:v]overlay -t 5 rtmp

Same file, same stream, but overlay doesn't work properly. Screen turns black with graphics on top:
ffmpeg -i udp -filter_complex "movie=ov3.webm:s=dv[ov];[0:v][ov]overlay,zmq" -t 5 rtmp

webm file was originally generated from ProRes MOV, which works well in both scenarios, but is too huge to be properly used for a long file. movie filter is mandatory for ZMQ to work.

Thank you all for your work and awesome talents that you dedicate for this magnificent video engineering apparatus!

[oscord]

works:

ffmpeg -f lavfi -i smptebars=size=1280x720 -c:v libvpx-vp9 -i ov3.webm -filter_complex "[0:v][1:v]overlay" -t 5 OK.mp4

fails:

ffmpeg -f lavfi -i smptebars=size=1280x720 -filter_complex "movie=ov3.webm:s=dv[ov];[0:v][ov]overlay" -t 5 NOT_OK.mp4